1. Field of the Invention
The present invention relates to a method and apparatus which employ transmitted light to continuously measure the individual cell size of cells contained in an extruded plastic foam product such as polystyrene foam. By continuously monitoring this foam characteristic, i.e. cell size, the variation of which has a pronounced effect upon the physical properties of the foam, prompt processing and/or material feed (e.g., blowing agent, nucleating agent, etc.) adjustments may be made to bring the cell size back into a specified range in the event that it deviates therefrom.
2. Description of the Prior Art
It is known in the prior art that during the extrusion of polymer foam, cell size is the structural parameter which can be modified most readily and, furthermore, has a pronounced effect on the foam's properties. These properties include compressive strength, tensile strength, elongation at break, tear strength and thermal insulating values. Cell size can be varied by orders of magnitude in a full scale production process by manipulating the necessary process parameters, nucleating agent, melt temperature, and so forth. The quantification of cell size is, under normal circumstances, tedious and time consuming. Normally a thin section of foam is prepared using a suitable cutting device, microtome or its equivalent. The sample is then examined under a microscope or other suitable device and an attempt is made to characterize the cell size by measuring the mean diameter of the cell or counting the number of cells contained in a given area. Obviously, these prior art methods do not lend themselves to continuous monitoring nor do they necessarily reflect the average cell size of the sample under investigation, since distribution of cell sizes occurs in most foam polymer systems and single point measurements cannot always be translated into an average measurement. In addition, these kinds of cell size quantification can be costly in a production situation. Poor quality and/or "off-specification" material can be manufactured for long periods of time while an out-of-line cell size measurement is being made. Accordingly, it is an object of the present invention to provide a continuous in-line method and apparatus for determining the cell size of polymeric foams as they are extruded. Further, the present invention provides an arrangement whereby in the event the monitored cell size of the foam is not within product specifications, automatic adjustments may be made to extrusion line conditions and/or to the extrusion feed materials to bring the cell size of the foam material within the desired specifications.